This application is related to U.S. patent application titled "Pair of Binary Diffraction Optics for Use in Over Filled Raster Output Scanning Systems" Ser. No 07/990,491, filed concurrently herewith (common assignee), now U.S. Pat. No. 5,315,427.
This invention relates to correction of the non-uniformity of the flux in a raster scanner system over one scan line caused by the rotation of a polygon mirror.
Referring to FIG. 1, a conventional raster scanner system utilizes a light source 12 and a multi-faceted rotating polygon mirror 14 as the scanning element. The light source 12, which can be a laser source, produces a light beam 16 and sends it to the polygon 14 which has a plurality of facets, each of which is a plane mirror. The raster scanner system utilizes overfill scanning and therefore, the light beam 16 is directed onto at least two facets 18 of the rotating polygon mirror 14. Facets of rotating polygon mirror 14 reflect the light beam 16 and also cause the reflected light 16 to revolve about an axis near the center of rotation of the rotating polygon mirror 14 scanning a line. This reflected light beam can be utilized to scan a document at the input 19 of an imaging system or can be used to impinge upon a photographic film or a photosensitive medium 19, such as a xerographic drum at the output of the imaging system.
Many of the conventional raster scanner systems, which use overfill scanning, exhibit a light beam which has a varying flux over one scan line. The flux of the light beam is higher at the start of the scan line than at the end of the scan line for a given rotation direction of the polygon. If the variation of the flux (difference between the highest flux and the lowest flux) becomes more than a certain percentage of the highest flux, it can be observed on the printed document as darker prints at one end of the document and as lighter prints at the other end of the document. The problem comes mainly from the facet aspect change caused by rotation of the polygon. When a facet rotates from the start of scan position to the end of scan position, it losses some energy as it leaves one position and gains some energy as it enters a subsequent position. Due to the changing aspect typically, the lost energy is larger than the gained energy. Therefore, as the facet rotates, the total energy received by the facet decreases and as a result the total flux of energy, also decreases. If the polygon direction of rotation were reversed, the facet would gain energy as the polygon rotated. This problem is exasperated by the nonuniform nature of typical laser beams over their intensity profile.